In recent years, flat display devices such as liquid crystal display devices have come to be used widely as image display devices of TV receivers, various computers, etc. because of their advantages that they are low in thickness, weight, and power consumption.
For example, transmissive liquid crystal display devices are equipped with a liquid crystal panel in which a liquid crystal layer is held between a pair of transparent substrates and a surface light source device which is disposed behind the liquid crystal panel (i.e., on the side opposite to an image display screen) and applies light to the liquid crystal panel.
To further reduce the thickness and the size of such liquid crystal display devices, it is necessary to decrease the thickness of the surface light source device further. To this end, side light type (edge light type) surface light source devices (backlights) have come to be employed more than direct light type ones.
Side light type surface light source devices have a linear light source and a thin-plate-shaped light guide plate which is made of an acrylic resin or the like and whose back surface is printed with a scattering pattern. The linear light source is disposed close to one side surface of the light guide plate. Light emitted from the linear light source travels through the light guide plate, is scattered by the scattering pattern of the back surface of the light guide plate, and exits from the liquid-crystal-panel-side main surface of the light guide plate.
Surface light source devices of liquid crystal display devices are increasingly required to be increased in size and decreased in thickness to accommodate size increase and thickness reduction of TV receivers. However, when the size of a surface light source device is increased, it is difficult to maintain its luminance and luminance uniformity and hence to decrease its thickness.